1. Field
Example embodiments relate to semiconductor memory devices using Trellis coded modulation (TCM), methods of storing data in the memory devices, and methods of reading data from the memory devices. Also, example embodiments relate to multi-level cell (MLC) memory devices using TCM, methods of storing data in the MLC memory devices, and methods of reading data from the MLC memory devices.
2. Description of Related Art
A single-level cell (SLC) memory device stores one bit of data in a single memory cell. The SLC memory device also is referred to as a single-bit cell (SBC) memory.
FIG. 1 illustrates a cell threshold voltage distribution of a SLC memory device according to related art. As shown in FIG. 1, the SLC memory device stores and reads data of one bit at a voltage level included in two distributions divided by a threshold voltage level programmed in a memory cell. For example, when a voltage level read from the memory cell is greater than 0.5 volts and less than 1.5 volts, it is determined that the data stored in the memory cell has a logic value of “1”. When the voltage level read from the memory cell is greater than 2.5 volts and less than 3.5 volts, it is determined that the data stored in the memory cell has a logic value of “0”. The data stored in a memory cell is classified depending on the difference(s) between cell currents and/or cell voltages during the reading operations.
Meanwhile, a MLC memory device that can store data of two or more bits in a single memory cell has been proposed in response to a need for higher integration of memory. The MLC memory device also is referred to as a multi-bit cell (MBC) memory. However, as the number of bits stored in the single memory cell increases, reliability deteriorates and read-failure rate increases. To store ‘m’ bits in a single memory cell, 2m voltage level distributions are required. But since the voltage window for a memory cell is limited, the difference in threshold voltage between adjacent bits decreases as ‘m’ increases, causing the read-failure rate increases. For this reason, it is not easy to improve storage density using a MLC memory device.
Accordingly, it may be desirable to develop MLC memory devices, methods of storing data in the MLC memory devices, and/or methods of reading data from the MLC memory devices.